Cleansing your subscriber list of "bad" email address' is needed for email marketing success. Mailing to invalid email address' will keep your bounce rates at a high level. When ISP's notice these high bounce rates your account may be blacklisted. Actively monitoring your list(s) and following a few good exercises to practice that will aid in keeping your list clean:
Create an Easily Accessible Change of Address Page
Subscribers change their email address' for several reasons and if they don't have an easy way to access a change of address form, they will not report that information and you will likely send mails to that invalid email address. This will cause the email to bounce and your reputation will take a hit. An accessible change of address page will increase the chances that the user will provide you with their new email address and other information if they want to continue to be subscribed. Even if the prospects are unlikely that the user will fill out a change of address form, it's a better idea to have the option.
Remove Hard Bounces
When you have hard bounces that means your emails are not getting delivered or there are issues with those address. Rid your list of those dead/inactive email addresses, and make an active effort to retrieve quality data in the future. When you perform this cleanse, the chances of your emails landing in the inbox will dramatically increase.
Data Entry Errors
When receiving handwritten email address' or when potential subscribers opt-in to your site for the first time, spelling mistakes may happen. Misspelled email address' lead to bounced emails and will negatively affect your results. There are also people who purposely misspell or use fake email addresses out there that need to be cleansed from your list. Actively, searching out these misspelled email address' or using a data verification service is recommended to keep these subscribers off your list.